In complex assemblies, even miniscule movements can have an incredible impact on performance. This movement, called backlash, can be addressed in an assembly by using ball screws or ACME screws manufactured through the proper process. While screw type is often determined on a case-by-case basis, several broad factors should be considered when making that decision.
Backlash is the excess play and lash of the nut on a screw in the axial direction. For example, if backlash is specified as .001 inches, the nut is allowed a maximum of .001 inches of travel on the screw without rotating the screw or nut.
Backlash is crucial in assemblies that require precise positioning. If an assembly has excessive backlash, then the positioning will not be as precise as required. This is often crucial in the aerospace and medical industries, where a tight positioning tolerance is needed to maintain the efficiencies in their products. In addition to positioning requirements, excessive backlash may also cause the nut may also be noisy when traveling, which results in a less smooth operation.
Certain assemblies require preloaded nuts, which do not have backlash. However, the more preload that an assembly has, the more torque the motor will need to turn the screw.
Backlash and Screw Type
While often backlash is associated with ball screws, backlash requirements have become more common for ACME screws in recent years. Each screw type resolves backlash requirements differently.
Consistent ball screw backlash can be achieved by utilizing a tighter lead tolerance screw. Using a lower grade ball screw, manufactured via thread grinding, can help achieve this. If cost is also a limiting factor, higher grade ball screws manufactured by thread rolling may also be able to achieve backlash requirements, though the screw will be less accurate. In addition, adjusting the ball bearing size in the ballnut may help, depending on the required backlash.
In ACME screws, the threads are the determining factor for backlash. To ensure the proper backlash is achieved, the PST Group matches a screw to a nut in the manufacturing process. Typically, the nut is threaded first, then the screw is thread ground to a specific size depending on the requirements.
While these guidelines can be helpful in starting the conversation regarding backlash requirements, the decision of which screw should be used to achieve proper backlash must be made on a case-by-case basis. The PST Group is currently working with an aerospace customer that was faced with this decision. While ball screws are almost always used in a particular application for this customer, the team at PST recommended to utilize a low-backlash ACME screw instead, due to the customer’s tight budget. By finding the proper ACME screw and nut assembly, the team was able to meet the customer’s financial demands, while still ensuring proper backlash for the application.
How PST Can Help
The PST Group has the ability to customize applications to meet any goals or challenges the customer presents. Even if backlash requirements are out of the ordinary, the PST team is able and willing to investigate and engineer a solution. Ready to get started? Contact us.